It is very disheartening to hear of all your problems. This is obviously a big disappointment for you. While every car has its problems, you seem to have had them all in a single purchase.
There are certainly a few "chronic" issues with the GXP, but by far yours seems to be the worst. I'm not really sure what to suggest. Have you been to the same dealer for all the service? I found for example, that a Cadillac dealer had far more experience with engine related problems. There are now more Pontiacs with the Northstar engine, but for a while the Bonneville was the only car in this line. Any of the other body type issues should be fixable by any dealership.
I've had my oil cooler lines replaced twice and my transmission lines once. A compass module, water pump and intermediate steering shaft. Brakes have been done, but only due to normal wear. The front main seal on the engine has also been replaced.
These cars are also known for some water leaks. These are often solvable with a trip through our techinfo section and searches within the forum. Several people have great write-ups here to help with this.
The fog light harness is a problem (although mine has been fine). I generally don't drive with my fog lights on unless there is fog, so my lights see very limited use. There have been a couple of reports with sunroof problems, although not extremely frequent. Some of the other issues you speak of (valve cover seals for example) don't seem to be common issues.
Again, very sorry to hear of all your problems. These are otherwise very nice cars to drive. I find mine extremely comfortable on the highway. Even with the problems I've seen, I wouldn't get rid of mine (although I've had significantly fewer issues).
StingRay Bike w/cards in the spokes, Weeeeeee!